Conversion and Deconversion

Conversion plants are operating commercially in the USA, Canada, France, Russia and China.

Deconversion of depleted UF6 to uranium oxide or UF4 is undertaken for long-term storage of depleted uranium in more stable form.

Uranium leaves the mine as the concentrate of a stable oxide known as U3O8 or as a peroxide. It still contains some impurities and prior to enrichment has to be further refined before or after being converted to uranium hexafluoride (UF6), commonly referred to as 'hex'. Both processes are normally included in the step between the mine and enrichment plant – referred to as 'conversion'.

Conversion plants are operating commercially in the USA, Canada, France, Russia and China. The main new plant is Areva’s Comurhex, operating between two sites in France. China’s capacity is expected to grow considerably through to 2025 and beyond to keep pace with domestic requirements.

The main process involves dissolving the oxide concentrate in nitric acid to produce uranyl nitrate, purifying this, evaporation of the purified uranium stream and thermal decomposition to uranium trioxide (UO3) powder. This ‘wet process’ process is used by Cameco in Canada, by Areva in France and also at Lanzhou in China and Seversk in Russia.

The uranium trioxide is reduced in a kiln by hydrogen to uranium dioxide. This is then reacted in another kiln with hydrogen fluoride (HF) to form uranium tetrafluoride (UF4). The tetrafluoride is then fed into a fluidised bed reactor with gaseous fluorine to produce UF6. This three-step process minimises fluorine demand.

The alternative ‘dry process’ used in the USA proceeds by a dry fluoride volatility process at high temperature straight to UF6, which is then refined.

Some secondary supplies, from downblended high-enriched uranium or re-enriched tails (see below) may be supplied or already exist in the form of UF6. Recycled uranium from reprocessing plants needs to be converted so that it can be enriched.

Secondary supplies of conversion are about 12,000 tU/yr in 2015.

Chemistry of conversion

In the dry process, uranium oxide concentrates are first calcined (heated strongly) to drive off some impurities, then agglomerated and crushed.

For the wet process, the concentrate is dissolved in nitric acid. The resulting clean solution of uranyl nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined in a fluidised bed reactor to produce UO3 (or UO2 if heated sufficiently).

Alternatively, the uranyl nitrate may be concentrated and have ammonia injected to produce ammonium diuranate, which is then calcined to produce pure UO3.

Crushed U3O8 from the dry process and purified uranium oxide UO3 from the wet process are then reduced in a kiln by hydrogen to UO2:

U3O8 + 2H2 ===> 3UO2 + 2H2O deltaH = -109 kJ/mole

or UO3 + H2 ===> UO2 + H2O deltaH = -109 kJ/mole

This reduced oxide is then reacted in another kiln with gaseous hydrogen fluoride (HF) to form uranium tetrafluoride (UF4), though in some places this is made with aqueous HF by a wet process:

UO2 + 4HF ===> UF4 + 2H2O deltaH = -176 kJ/mole

The tetrafluoride is then fed into a fluidised bed reactor or flame tower with gaseous fluorine to produce uranium hexafluoride, UF6. Hexafluoride ('hex') is condensed and stored.

UF4 + F2 ===> UF6

Removal of impurities takes place at each step.

At Converdyn’s US conversion plant, U3O8 is first made into impure UF6 and this is then refined in a two-stage distillation process.

The UF6, particularly if moist, is highly corrosive. When warm it is a gas, suitable for use in the enrichment process. At lower temperature and under moderate pressure, the UF6 can be liquefied. The liquid is run into specially designed steel shipping cylinders which are thick walled and weigh over 15 tonnes when full. As it cools, the liquid UF6 within the cylinder becomes a white crystalline solid and is shipped in this form.

The siting, environmental and security management of a conversion plant is subject to the regulations that are in effect for any chemical processing plant involving fluorine-based chemicals.

Secondary sources of conversion supply

Secondary supply of equivalent conversion services includes UF6 material from commercial and government inventories, enricher underfeeding, and DU tails recovery. Uranium and plutonium recycle effectively adds to this. All these were estimated at 26,000 tU in 2013 but with the end of the Russian HEU supply to USA, they are much less from 2014, and are projected to be less than 14,000 tU to 2022.

Depleted uranium and deconversion

Up to 90% of the original uranium feed ends up as depleted uranium (DU), which is stored long-term as UF6 or preferably, after deconversion, as U3O8, allowing HF to be recycled. It may also be deconverted to UF4, which is more stable, with much higher temperature of volatalisation. To early 2007, about one-quarter of the world's 1.5 million tonnes of DU had been deconverted. World deconversion capacity was about 60,000 t/yr at end of 2010.

The main deconversion plant is the 20,000 t/yr one run by Areva NC at Tricastin, France, and over 300,000 tonnes has been processed here. The technology has been sold to Russia. Two plants have been built by Uranium Disposition Services (UDS) at Portsmouth and Paducah, USA, with capacities of 13,500 and 18,000 t/yr respectively. A 6500 t/yr plant is being built at New Mexico in the USA by International Isotopes (INIS). In the UK, Urenco ChemPlants is building a plant and expects 2017 commissioning.

Uranium Deconversion Plants

Operator

Location

Capacity tU/yr

Areva

Tricastin, France

20,000

Richland, Washington, USA

small

Urenco ChemPlants

Capenhurst, UK

15,000

Mid America Conversion Services

Portsmouth, Ohio, USA

13,500

Paducah, Kentucky, USA

18,000

INIS Fluorine Products

Hobbs, New Mexico, USA

6,500 (construction on hold)

Tenex

Zelenogorsk, Russian Federation

10,000

Russia’s W-ECP deconversion plant is at Zelenogorsk Electrochemical Plant (ECP) in Siberia. The 10,000 t/yr deconversion (defluorination) plant was built by Tenex under a technology transfer agreement with Areva NC, so that depleted uranium can be stored long-term as uranium oxide, and HF is produced as a by-product. The W-ECP plant is similar to Areva’s W2 plant at Pierrelatte in France and has mainly west European equipment. It was commissioned in December 2009.

Russia is also building a plant at Angarsk to deconvert UF6 to UF4, recovering some HF in the process. Capacity of 2000 t/yr was planned for 2012, with subsequent increase to 6000 t/yr.

These use essentially a dry process, with no liquid effluent. It is the same as that used for the enriched portion, albeit at a scale of 20,000 tonnes per year in the one plant.

The UF6 is first vapourised in autoclaves with steam, then the uranyl fluoride (UO2F2) is reacted with hydrogen at 700°C to yield an HF product for sale to converters and U3O8 powder which is packed into 10-tonne containers for storage.

UF6 + 2H2O ==⇒ UO2F2 + 4HF

3UO2F2 + 2H2O + H2 ===> U3O8 + 6HF

The INIS plant in Idaho uses a slightly different deconversion followed by fluorine extraction process (FEP), on a toll basis. The deconversion plant had been used to produce DU metal for the military and was purchased by INIS. In this, the depleted UF6 is first vapourised in autoclaves and hydrogen is added to give depleted UO2 and anhydrous UF4 which is the main product for sale. The FEP then involves reacting some UF4 with silica to give silicon fluoride (SiF4) as a commercial co-product.

Ownership title is normally transferred to the enricher as part of the commercial deal. It is sometimes considered as a waste, though only for legal or regulatory reasons and in the USA, but usually it is understood as a long-term strategic resource which can be used in a future generation of fast neutron reactors. Any much more efficient enrichment process would also make it into an immediately usable resource to supply more U-235. Enrichment companies with ownership of large amounts of depleted uranium are quite clear that their stocks are a significant asset, though Urenco speaks of deconversion being for long-term storage “prior to geological disposal”.